1. Field of the Invention
The present invention relates to an electrophotographic image forming process and image forming apparatus which effectively inhibit separation electrification (contact electrification) between a surface layer of a belt member and an image-receiving layer of an electrophotographic image-receiving sheet at a cooling and separating unit, prevent adsorption of dust to charges on the belt and the surface of the electrophotographic image-receiving sheet, and allow printing of a high quality image having a near-photograph quality.
2. Description of the Related Art
A problem in endless belt fixing is that separation electrification is induced at a belt surface layer and an image-receiving layer of an electrophotographic image-receiving sheet at a cooling and separating unit, and failures by dust adsorption to the charges at the layer surfaces are likely to occur. Particularly, the dust adhered to the belt may subsequently cause defects in image-receiving sheets and has possibility to cause a significant failure which occurs repeatedly at the same spot, and therefore a solution of this problem is desired.
For example, Japanese Patent Application Laid-Open (JP-A) No. 03-25476 discloses a fixing device that fixes a toner image by applying heat to the toner image indirectly through a film, in which the film has multiple layers each of which has a volume resistivity of 1011 Ω·cm or less so that the layer that slides over a heater is maintained substantially at a predetermined electric potential.
JP-A No. 03-242673 discloses a fixing device which has a sheet-shaped member such as a heat resistant film and a driving roller which drives this sheet-shaped member. The fixing device heats a developed image on a recording material with heat from a heater through the sheet-shaped member. The driving roller includes a metal roller and an elastic surface layer containing a conductive material coated on the metal roller, and a volume resistivity of the elastic surface layer is 1011 Ω·cm or less.
JP-A No. 04-51156 discloses a thermal fixing process that fixes a developed image formed by toner with electrophotography to a recording material, in which a surface resistivity of a film between a heater and a pressure applier is 1015 Ω/cm2 or less.
JP-A No. 08-63017 discloses an image heating process in which a film having a conductive layer is used, and an eddy current is generated in the conductive layer of the film upstream of a nip to generate heat and to heat a toner image, and then after the temperature of the toner becomes lower than its glass transition point, the recording material on which the toner image is formed is separated from the film.
JP-A No. 09-190099 discloses a fixing device having a fixing roller; a driven roller; a heating belt which is mounted over the driven roller and the fixing roller; a pressuring roller which is arranged oppositely to the fixing roller and which forms a nip with the heating belt, the nip constituting a first fixing unit; and a heating source which is arranged at the fixing roller and/or the pressuring roller. The heating belt has a conductive member made of nickel or the like as a base, and a releasing material layer which is arranged on the outside of the base and contains a fluorine resin.
JP-A No. 2001-302812 discloses an endless belt whose surface resistivity is from 1×10 Ω to 1×1016 Ω or whose volume resistivity is from 1×10 Ω·cm to 1×1016 Ω·cm. The belt is to be used in an image forming apparatus as an intermediate transfer belt, conveyor transfer belt, or photoconductor belt.
Although JP-A Nos. 03-25476, 03-242673, and 04-51156 describe electric resistance values of fixing films, they do not describe electrophotographic image-receiving sheet at all, and moreover, they do not disclose nor imply a cooling device nor cooling separation. JP-A Nos. 08-63017 and 09-190099 do not describe specific values of conductivity, and although JP-A No. 2001-302812 describes an endless belt and defines the electric resistance thereof, the belt is not for use as a fixing belt.
At any rate, the above-mentioned disclosures do not disclose nor imply separation electrification at the time of separation, and since the separation electrification is generated between an electrophotographic image-receiving sheet and an endless belt, it is difficult to prevent dust adsorption failures by the disclosures.
When a high quality image having a near-photographic quality is to be printed, it is effective to use an electrophotographic image-receiving sheet which has polymer layers on both sides, but polymer layers are generally insulators and particularly likely to cause separation electrification, which is a significant problem. To prevent this, electric properties have to be given for both the belt surface and the electrophotographic image-receiving sheet, which determine the amount charge by separation electrification, but in prior art, no consideration has been made.